Alluvial Sands 3D ERI

Fig. 1. 3D subsurface resistivity from ERI of river-terrace gravelly sand over bedrock in Lincolnshire UK. High resistivities(orange-red) are confirmed moderately to well sorted gravelly sand. Low resistivities(blues) under the gravelly sand are confirmed mudstone. Moist, fine topsoil also exhibits low resistivities. Note spatial variability of gravelly sand resistivity. Dashed white line at ~ 9 m depth is the isosurface in the resistivity model that corresponds to the resistivity value in the basal gravel at an excavation to bedrock in field center.*

Fig. 2. Photograph of area (120 m x189 m) in Lincolnshire UK surveyed for 3D ERI showing the survey grid (blue lines) and location of two boreholes (red crosses). Survey lines parallel to the y-axis were separated by 6 m; lines parallel to the x-axis were separated by 12 m. Two additional survey lines were positioned at y= 6 and 186 m to improve resolution at the margins.*

Fig. 3. Resistivity values (black circles) and interpolated curve from the 3D ERI data at x=77m, y=100 m (location of excavation to bedrock) versus depth. Interface resistivity at ~9 m depth (147 Ωm) is the resistivity value that coincides with the basal gravel at the excavation to bedrock. Use of this value to pick deposit contact in resistivity model is the “known interface method” (KIM) for edge detection. Grey pts and curve give the sensitivity of the inverted resistivity model. Sensitivity of the model decreases with depth or increasing distance from surface electrodes.*